Screw rotors for rotary screw machines are known in the art. Each rotor has helically extending lobes and intermediate grooves, through which the rotors intermesh, one rotor is a male rotor with each lobe in a section perpendicular to the rotor axes having a leading lobe flank and a trailing lobe flank, both being substantially convex. The other rotor is a female rotor with each lobe in said section having a leading and a trailing lobe flank, both being substantially concave. Each lobe of the male and female rotor has an asymmetric profile in said section.
In a rotary screw machine of the kind for which the rotors of the invention are intended, a compressible medium is compressed or expanded by intermeshing two rotors in a working space sealingly surrounding the pair of rotors which has the shape of two intersecting circular cylinders.
Decisive to the function and the efficiency of such a machine is the shape of its rotors, more precisely the shape of the flanks of the rotor lobes.
Normally, in a rotary screw compressor at work only one of the rotors is driving and transmits torque to the other one, the driven rotor. Usually a liquid is injected such as oil or water into the working space of the machine, which liquid forms a film on the flanks of the lobes for lubricating, cooling and sealing purposes. The lobes co-operate by intermeshing and are shaped to transmit torque between the rotors and to seal working chambers in the working space of the machine. An important aspect when designing the profiles of the lobes therefore is to attain a contact band between the rotors that in this respect is optimal. The contact band should be of sufficient size for the contact pressure which the material and the liquid film are exposed to. When designing the rotor profiles one has to take the total length of the contact band or the sealing line into consideration as well as other general aspects such as the size of the blow-hole, the contact forces, the volumetric capacity, thermal expansion, generation of vibrations and demands relating to the manufacture. There are also some mathematical limitations for the profiles. For some compressors, certain aspects are more important than others and for other compressors there might be reasons to give priority to other aspects. An optimal profile usually represents a compromise between different requirements related to these aspects, the compromise being dependent on which of these are the most important in the actual case.
Due to the decisive importance of the shape of the rotor profiles in a rotary screw machine and due to the complex weighting between the aspects that have to be considered there are a large number of granted patents focusing on the profiles, all since Lysholm during the thirties presented and got a patent for the first rotary screw compressor of this kind that could be used in practice.
There are many ways in patent literature in which the rotor profiles are defined, depending on which problem(s) the patent relates to and due to the complicated shape of these profiles. The profiles are thus defined as a family of characteristics, a combination of such, by some important parameters, by ranges for certain features of the profile, by expressions implicitly defining the profile or in another way. Furthermore the profiles can be divided into different categories according to various criteria such as symmetric or asymmetric profiles and such as point generated or line generated.
It is understood that point generation refers to that a single point on either of the rotors generate a longer part on the other of the two rotors, and that line generation refers to that one single point on one rotor corresponds only to one single point on the other of the two rotors. Point generation may be disadvantageous since a manufacturing error or wear at the generation point on one of the rotors will open a leakage along the entire generated part on the other of the rotors. Line generation does not suffer from this problem, but may on the other increase drag losses and friction.
U.S. Pat. No. 3,423,017 discloses an asymmetric profile, the so called “A-profile”, which is line generated on the leading flank and point generated on the trailing high pressure flank. The blow hole is substantially reduced compared to earlier profiles due to the use of reciprocal point generation on the high pressure flank. The torque transmission characteristics are furthermore advantageous. One problem however is that this profile is difficult and/or expensive to manufacture in manufacturing tools, due to its relatively sharp corners and closed shape.
U.S. Pat. No. 4,435,139 discloses another asymmetric profile, the so called “D-profile”, which is easier to manufacture, but on the other provides less advantageous torque transmission characteristics and high surface pressure at the contact band.
U.S. Pat. No. 5,947,713 discloses yet another profile, the so called “G-profile”, which aims to solve the problem of high surface pressure by providing the two rotors with arc segments of corresponding radius. This profile is however also quite closed in character and may therefore be difficult and/or expensive to manufacture.